1. Technical Field
This invention relates to audio transducers. More particularly, this invention relates to lightweight, audio transducers.
2. Related Art
Electrodynamic loudspeakers include a diaphragm connected to a voice-coil. The voice-coil is positioned in an air gap between the poles of a magnet. The magnets produce magnetic flux in the air gap. These magnets are typically permanent magnets and are used in a magnetic circuit of ferromagnetic material to direct the flux produced by the permanent magnet into the air gap.
The voice-coil is placed in the air gap with its conductors wound substantially cylindrically so as to be placed perpendicular to the main component of the magnetic flux in the air gap. The coil is then connected mechanically to a loudspeaker diaphragm that is driven or vibrated by the axial motion of the voice-coil produced by the motor force on the voice-coil when it is connected to an audio amplifier. The coil is referred to the “voice” coil because, in loudspeakers or similar electromechanical transducers, the frequency range of interest is in the extended range of the human voice.
The voice-coil is normally connected to an audio amplifier of some type that produces a current in the voice-coil that is a function of the electrical signal to be transformed by the loudspeaker into an audible, sub-audible or ultrasonic pressure variation. The voice-coil is intended to carry a current in a direction that is substantially perpendicular to the direction of the lines of magnetic flux produced by the permanent magnet. The magnetic structure is often arranged to provide cylindrical symmetry with an annular air gap in which the magnet flux lines are directed radially with respect to the axis of cylindrical symmetry of the loudspeaker.
Permanent-magnet electro-dynamic loudspeakers employ a diaphragm that is vibrated by an electromechanical drive. The drive generally includes a motor structure comprised of one or more magnets plus ferrous material, and a voice-coil with an electrical signal passed through the voice-coil. The interaction between the current passing through the voice-coil and the magnetic field produced by the permanent magnet causes the voice-coil to oscillate in accordance with the electrical signal and, in turn, drives the diaphragm and produces sound.
In loudspeaker magnet systems, ferrous pole material is employed to create the gap and to guide the magnetic field, i.e., create the magnetic circuit. An axially magnetized magnet is positioned in a ferrous cylinder so that one pole of the magnet is in contact with bottom of the cylinder. The diameter of the magnet is less than that of the cylinder such that there is created an annular gap between the lateral sides of the magnet and interior walls of the cylinder. A second ferrous material, such as a disk that is roughly the same diameter as the magnet, is placed on top of the magnet so as to be in contact with the opposing pole of the magnet. The cylinder focuses the magnetic flux from the magnetic pole with which it is in contact and disk. One or multiple axially magnetized magnets may be included in such systems.
These ferrous materials may contribute a significant portion of the total mass of the system. Ferrous systems also may increase voice-coil inductance. Thus, as frequency increases, voice-coil inductance increases, resulting in reduced speaker output. Further, in operation, the resistance of the conductive material of the voice-coil causes the production of heat in the voice-coil or winding. The presence of ferromagnetic material may also contribute to an increased production of heat.
The problems produced by heat generation are further compounded by temperature-induced resistance, commonly referred to as power compression. As the temperature of the voice-coil increases, the DC resistance of copper or aluminum conductors or wires used in the voice-coil also increases. For example, a copper wire voice-coil that has a resistance of six ohms at room temperature has a resistance of twelve ohms at 270 degree C. (520 degree F.) At higher temperatures, power input is converted mostly into additional heat rather than sound, thereby seriously reducing loudspeaker efficiency.
Thus, heat production is a major determinant of loudspeaker maximum sound pressure output. Thus, devices may be limited in their maximum sound pressure because of the heat they generate. In a typical single voice-coil design using a ceramic magnet, the loudspeaker is very large and a heat sink is usually not employed. As such, because the driver must not overheat, the maximum allowable temperature limits the input power capacity of the loudspeaker. A common approach in the design of high power professional loudspeakers consists of simply making the motor structure large enough to dissipate the heat generated in the voice-coil. Producing a high power loudspeaker in this way results in a very large and heavy loudspeaker with a large motor structure. These large and heavy loudspeakers may not be feasible for use in vehicular applications due to weight and space limitations.
Thus, there is a need for loudspeaker systems that dissipate the heat generated by the voice-coil, thus, improving efficiency and producing greater power output. It may also be desirable to have a magnetic field system that is constant in a region and drops to a low value outside the region. Therefore, a need exists for a magnetic field system that can produce a desired magnetic field distribution without the use of any ferrous pole material.